trie/bintrie: simplify StemNode to array-of-slices representation

Gballet asked on PR #34055 (comments 3100043116, 3100050542, and the
bit-check dedup at 3100114416 / 3100878310) to revert StemNode from the
packed-bytes representation to the straightforward array-of-slices.

Before: StemNode carried a bitmap, a concatenated valueData []byte, a
count, and a shared COW flag. Every read/write went through a bit-count
posInData lookup; every mutation through ensureWritable COW.

After: values [StemNodeWidth][]byte — 256 slots, nil == absent. No
bitmap lookup, no COW. Direct sn.values[suffix] access.

Supporting changes:
- Drop posInData, ensureWritable; rewrite getValue/hasValue/allValues/
  setValue as trivial slice access.
- Hash() iterates sn.values directly, matching master's shape.
- SerializeNode emits the bitmap + concatenated bytes on the wire from
  the array-of-slices at serialize time; wire format unchanged.
- decodeNode populates sn.values[i] slots by aliasing the serialized
  buffer (zero-copy).
- NodeStore.Copy deep-copies each slot.
- splitStemValuesInsert + the insertSingleInternal paths write directly
  to sn.values[i].

Trade-off: stems now carry 256 []byte headers (6144 B) instead of 1
concatenated slice (~32 B) + bitmap. Stem-pool scan cost returns to
parity with master (the existing valueData pointer already made the
pool non-noscan; rollback adds 255 more pointers per stem). The primary
arena win — pointer-free InternalNode pool — is preserved.

trie/bintrie/node_store.go

@@ -170,12 +170,18 @@ func (s *NodeStore) Copy() *NodeStore {
 		cp := *chunk
 		ns.stemChunks[i] = &cp
 	}
+	// Deep-copy each stem's value slots — they may alias serialized buffers,
+	// so we can't rely on the chunk-wise struct copy above.
 	for i := uint32(0); i < s.stemCount; i++ {
 		src := s.getStem(i)
 		dst := ns.getStem(i)
-		if len(src.valueData) > 0 {
-			dst.valueData = make([]byte, len(src.valueData))
-			copy(dst.valueData, src.valueData)
+		for j, v := range src.values {
+			if v == nil {
+				continue
+			}
+			cp := make([]byte, len(v))
+			copy(cp, v)
+			dst.values[j] = cp
 		}
 	}
 	ns.hashedChunks = make([]*[storeChunkSize]HashedNode, len(s.hashedChunks))

trie/bintrie/stem_node.go

@@ -17,106 +17,40 @@
 package bintrie
 
 import (
-	"math/bits"
-
 	"github.com/ethereum/go-ethereum/common"
 )
 
-// StemNode holds up to 256 values sharing a 31-byte stem, packed via bitmap.
+// StemNode holds up to 256 values sharing a 31-byte stem.
 //
 // Invariant: dirty=false implies mustRecompute=false. Every mutation that
 // invalidates the cached hash MUST also mark the blob for re-flush.
 type StemNode struct {
-	Stem      [StemSize]byte
-	bitmap    [StemBitmapSize]byte
-	valueData []byte
-	count     uint16
-	depth     uint8
-	shared    bool // true if valueData is shared with serialized input
+	Stem   [StemSize]byte
+	values [StemNodeWidth][]byte // nil == slot absent
+
+	depth uint8
 
 	mustRecompute bool        // hash is stale (cleared by Hash)
 	dirty         bool        // on-disk blob is stale (cleared by CollectNodes)
 	hash          common.Hash // cached hash when mustRecompute == false
 }
 
-// posInData returns the offset within valueData, or -1 if absent.
-func (sn *StemNode) posInData(suffix byte) int {
-	idx := int(suffix)
-	if sn.bitmap[idx/8]>>(7-(idx%8))&1 == 0 {
-		return -1
-	}
-	// Count the bits set before this position to determine the offset
-	pos := 0
-	byteIdx := idx / 8
-	for i := 0; i < byteIdx; i++ {
-		pos += bits.OnesCount8(sn.bitmap[i])
-	}
-	// Count bits in the partial byte
-	mask := byte(0xFF) << (8 - (idx % 8))
-	pos += bits.OnesCount8(sn.bitmap[byteIdx] & mask)
-	return pos
-}
-
 func (sn *StemNode) getValue(suffix byte) []byte {
-	pos := sn.posInData(suffix)
-	if pos < 0 {
-		return nil
-	}
-	start := pos * HashSize
-	return sn.valueData[start : start+HashSize]
+	return sn.values[suffix]
 }
 
 func (sn *StemNode) hasValue(suffix byte) bool {
-	idx := int(suffix)
-	return sn.bitmap[idx/8]>>(7-(idx%8))&1 == 1
+	return sn.values[suffix] != nil
 }
 
-// allValues returns all 256 values (nil for absent positions).
+// allValues returns the underlying slot array as a slice. nil entries mean
+// absent. Callers must treat it as read-only.
 func (sn *StemNode) allValues() [][]byte {
-	values := make([][]byte, StemNodeWidth)
-	dataIdx := 0
-	for i := range StemNodeWidth {
-		if sn.bitmap[i/8]>>(7-(i%8))&1 == 1 {
-			values[i] = sn.valueData[dataIdx*HashSize : (dataIdx+1)*HashSize]
-			dataIdx++
-		}
-	}
-	return values
-}
-
-// ensureWritable copies valueData if shared (copy-on-write).
-func (sn *StemNode) ensureWritable() {
-	if sn.shared || cap(sn.valueData)-len(sn.valueData) < HashSize {
-		newData := make([]byte, len(sn.valueData), len(sn.valueData)+HashSize*4)
-		copy(newData, sn.valueData)
-		sn.valueData = newData
-		sn.shared = false
-	}
+	return sn.values[:]
 }
 
 func (sn *StemNode) setValue(suffix byte, value []byte) {
-	sn.ensureWritable()
-	idx := int(suffix)
-	pos := sn.posInData(suffix)
-	if pos >= 0 {
-		copy(sn.valueData[pos*HashSize:], value[:HashSize])
-		return
-	}
-	sn.bitmap[idx/8] |= 1 << (7 - (idx % 8))
-	sn.count++
-
-	insertPos := 0
-	byteIdx := idx / 8
-	for i := 0; i < byteIdx; i++ {
-		insertPos += bits.OnesCount8(sn.bitmap[i])
-	}
-	mask := byte(0xFF) << (8 - (idx % 8))
-	insertPos += bits.OnesCount8(sn.bitmap[byteIdx] & mask)
-
-	insertOffset := insertPos * HashSize
-	sn.valueData = append(sn.valueData, make([]byte, HashSize)...)
-	copy(sn.valueData[insertOffset+HashSize:], sn.valueData[insertOffset:len(sn.valueData)-HashSize])
-	copy(sn.valueData[insertOffset:], value[:HashSize])
+	sn.values[suffix] = value
 }
 
 func (sn *StemNode) Hash() common.Hash {
@@ -128,28 +62,21 @@ func (sn *StemNode) Hash() common.Hash {
 	h := newSha256()
 	defer returnSha256(h)
 
-	// Hash each present value
-	dataIdx := 0
-	for i := range StemNodeWidth {
-		if sn.bitmap[i/8]>>(7-(i%8))&1 == 1 {
-			v := sn.valueData[dataIdx*HashSize : (dataIdx+1)*HashSize]
+	for i, v := range sn.values {
+		if v != nil {
 			h.Reset()
 			h.Write(v)
 			h.Sum(data[i][:0])
-			dataIdx++
 		}
 	}
-	h.Reset()
 
 	for level := 1; level <= 8; level++ {
 		for i := range StemNodeWidth / (1 << level) {
-			h.Reset()
-
 			if data[i*2] == (common.Hash{}) && data[i*2+1] == (common.Hash{}) {
 				data[i] = common.Hash{}
 				continue
 			}
-
+			h.Reset()
 			h.Write(data[i*2][:])
 			h.Write(data[i*2+1][:])
 			data[i] = common.Hash(h.Sum(nil))

trie/bintrie/store_commit.go

@@ -123,12 +123,26 @@ func (s *NodeStore) serializeNode(ref nodeRef) []byte {
 
 	case kindStem:
 		sn := s.getStem(ref.Index())
-		serializedLen := NodeTypeBytes + StemSize + StemBitmapSize + len(sn.valueData)
+		// Count present slots to size the blob.
+		var count int
+		for _, v := range sn.values {
+			if v != nil {
+				count++
+			}
+		}
+		serializedLen := NodeTypeBytes + StemSize + StemBitmapSize + count*HashSize
 		serialized := make([]byte, serializedLen)
 		serialized[0] = nodeTypeStem
 		copy(serialized[NodeTypeBytes:NodeTypeBytes+StemSize], sn.Stem[:])
-		copy(serialized[NodeTypeBytes+StemSize:NodeTypeBytes+StemSize+StemBitmapSize], sn.bitmap[:])
-		copy(serialized[NodeTypeBytes+StemSize+StemBitmapSize:], sn.valueData)
+		bitmap := serialized[NodeTypeBytes+StemSize : NodeTypeBytes+StemSize+StemBitmapSize]
+		offset := NodeTypeBytes + StemSize + StemBitmapSize
+		for i, v := range sn.values {
+			if v != nil {
+				bitmap[i/8] |= 1 << (7 - (i % 8))
+				copy(serialized[offset:offset+HashSize], v)
+				offset += HashSize
+			}
+		}
 		return serialized
 
 	default:
@@ -184,27 +198,25 @@ func (s *NodeStore) decodeNode(serialized []byte, depth int, hn common.Hash, mus
 		return ref, nil
 
 	case nodeTypeStem:
-		if len(serialized) < 64 {
+		if len(serialized) < NodeTypeBytes+StemSize+StemBitmapSize {
 			return emptyRef, errInvalidSerializedLength
 		}
 		stemIdx := s.allocStem()
 		sn := s.getStem(stemIdx)
 		copy(sn.Stem[:], serialized[NodeTypeBytes:NodeTypeBytes+StemSize])
-		copy(sn.bitmap[:], serialized[NodeTypeBytes+StemSize:NodeTypeBytes+StemSize+StemBitmapSize])
-
-		var count uint16
-		for i := range StemBitmapSize {
-			count += uint16(bits.OnesCount8(sn.bitmap[i]))
+		bitmap := serialized[NodeTypeBytes+StemSize : NodeTypeBytes+StemSize+StemBitmapSize]
+		offset := NodeTypeBytes + StemSize + StemBitmapSize
+		for i := range StemNodeWidth {
+			if bitmap[i/8]>>(7-(i%8))&1 != 1 {
+				continue
+			}
+			if len(serialized) < offset+HashSize {
+				return emptyRef, errInvalidSerializedLength
+			}
+			// Zero-copy: each slot aliases the serialized input buffer.
+			sn.values[i] = serialized[offset : offset+HashSize]
+			offset += HashSize
 		}
-		sn.count = count
-		dataStart := NodeTypeBytes + StemSize + StemBitmapSize
-		dataEnd := dataStart + int(count)*HashSize
-		if len(serialized) < dataEnd {
-			return emptyRef, errInvalidSerializedLength
-		}
-		// Zero-copy: aliases the serialized buffer; ensureWritable() COWs before mutation.
-		sn.valueData = serialized[dataStart:dataEnd]
-		sn.shared = true
 		sn.depth = uint8(depth)
 		sn.hash = hn
 		sn.mustRecompute = mustRecompute
@@ -279,13 +291,10 @@ func (s *NodeStore) toDot(ref nodeRef, parent, path string) string {
 		me := fmt.Sprintf("stem%s", path)
 		ret := fmt.Sprintf("%s [label=\"stem=%x c=%x\"]\n", me, sn.Stem, sn.Hash())
 		ret = fmt.Sprintf("%s %s -> %s\n", ret, parent, me)
-		idx := 0
-		for i := range StemNodeWidth {
-			if sn.bitmap[i/8]>>(7-i%8)&1 != 1 {
+		for i, v := range sn.values {
+			if v == nil {
 				continue
 			}
-			v := sn.valueData[idx*HashSize : (idx+1)*HashSize]
-			idx++
 			ret += fmt.Sprintf("%s%x [label=\"%x\"]\n", me, i, v)
 			ret += fmt.Sprintf("%s -> %s%x\n", me, me, i)
 		}

trie/bintrie/store_ops.go

@@ -479,9 +479,7 @@ func (s *NodeStore) insertValuesAtStem(ref nodeRef, stem []byte, values [][]byte
 		sn.dirty = true
 		for i, v := range values {
 			if v != nil {
-				sn.count++
-				sn.bitmap[i/8] |= 1 << (7 - (i % 8))
-				sn.valueData = append(sn.valueData, v[:HashSize]...)
+				sn.values[i] = v
 			}
 		}
 		return makeRef(kindStem, stemIdx), nil